The growth of demand for subscriber bandwidth has led to great pressure to expand the capacity of the telecommunications networks. Dense wavelength division multiplexing (DWDM) allows high bandwidth use of existing fiber, but low-cost cost components are required to enable provision of high bandwidth to a broad range of customers. Key components include the source, the detector, and routing components, but these components should preferably be addressable to any of the frequency channels. These channels are currently defined by the ITU as .nu..sub.n =.nu..sub.0.+-.n d.nu., where .nu..sub.0 is the central optical frequency 193.1 THz and d.nu. is the specified frequency channel spacing that may equal a multiple of 100 GHz or 50 GHz. Systems have also been demonstrated based on other fixed spacings, and based on nonuniform frequency spacings.
Semiconductor lasers with built-in gratings such as DFB and DBR lasers are currently used to produce the frequency-specific lasers needed to transmit over optical fibers. However, current fabrication techniques do not allow high yield production of a given frequency channel because of index of refraction variations in the InP-based materials. Because silica, polymer, and other optical materials offer greater stability of index of refraction, many types of hybrid lasers have been tested in which a semiconductor gain medium is combined with a grating fabricated in another material. Single frequency hybrid waveguide lasers have been demonstrated with semiconductor waveguide amplifiers to obtain the benefits of frequency selectivity and tunability. See for example * J. M. Hammer et al., Appl. Phys. Lett. 47 183, (1985), who used a grating in an external planar waveguide, by * E. Brinkmeyer et al., Elect. Lett 22 134 (1986) and * E. I. Gordon, U.S. Pat. No. 4,786,132, Dec. 22, 1988 and * R. C. Alferness, U.S. Pat. No. 4,955,028, Sep. 4, 1990, who used a grating in a fiber waveguide, by * D. M. Bird et al., Elect. Lett. 27 1116 (1991) who used a UV-induced grating, by * W. Morey, U.S. Pat. No. 5,042,898, Aug. 27, 1991 who used a fiber grating with thermally compensated package, by * P. A. Morton et al., Appl. Phys. Lett. 64 2634 (1994) who used a chirped grating, by * D. A. G. Deacon, U.S. Pat. No. 5,504,772, Apr. 2, 1996, who used multiple gratings with optical switches, by * J. N. Chwalek, U.S. Pat. No. 5,418,802, May 23, 1995, who used an electro-optic waveguide grating, by * R. J. Campbell et al., Elect. Lett. 32 119 (1996) who used an angled semiconductor diode waveguide, by * T. Tanaka et al, Elect. Lett. 32 1202 (1996) who used flip-chip bonding, and by * J-M. Verdiell, U.S. Pat. No. 5,870,417, Feb. 9, 1999, who adjust for single mode operation. Single frequency hybrid waveguide lasers have also been demonstrated with fiber waveguide amplifiers. See * D. Huber, U.S. Pat. No. 5,134,620, Jul. 28, 1992 and * F. Leonberger, U.S. Pat. No. 5,317,576, May 31, 1994.
Many robust thermo-optic materials are available today including glass and polymer materials systems that can also be used in fabricating waveguide optical components. See * M. Haruna et al., IEE Proceedings 131H 322 (1984), and * N. B. J. Diemeer, et al., J. Light. Technology, 7, 449-453 (1989). Recently, thermally tunable gratings have been fabricated in polymer waveguides and resonators. See * L. Eldada et al., Proceedings of the Optical Fiber Communications Conference, Optical Society of America, p. 98 (1999), and * N. Bouadma, U.S. Pat. No. 5,732,102, Mar. 24, 1998.
Thermal compensation of laser resonators is a requirement in components that must operate robustly within the narrow absolute frequency bands of the DWDM specifications. Thermally compensated resonators have has been shown using polymer materials. See * K. Tada et al., Optical and Quantum Electronics 16, 463 (1984). Thermally compensated packages for fiber grating based devices have also been shown. See * W. Morey, U.S. Pat. No. 5,042,898, Aug. 27, 1991, * G. W. Yoffe et al, Appl. Opt. 34 6859 (1995), and * J-M. Verdiell, U.S. Pat. No. 5,870,417, Feb. 9, 1999. Thermally compensated waveguides using mixed silica-polymer materials have also been shown to produce temperature independent characteristics. See * Y. Kokubun et al., IEEE Photon. Techn. Lett. 5 1297 (1993), and * D. Bosc, U.S. Pat. No. 5,857,039, Jan. 5, 1999. Silica-polymer waveguides have also been used for interconnecting laser devices. See * K. Furuya U.S. Pat. No. 4,582,390, Apr. 15, 1986.
The grating assisted coupler is a useful device for frequency control. Grating assisted couplers as described in * R. C. Alferness, U.S. Pat. No. 4,737,007, Apr. 12, 1988, are known in many configurations including with mode lockers, amplifiers, modulators, and switches. See * A. S. Kewitsch, U.S. Pat. No. 5,875,272, Feb. 23, 1999. Grating assisted couplers have been used in resonators including lasers, mode lockers, etalons, add-drop filters, frequency doublers, etc. See for example * E. Snitzer, U.S. Pat. No. 5,459,801, Jan. 19, 1994, and * D. A. G. Deacon, U.S. Pat. No. 5,581,642, Dec. 3, 1996.
What is needed is a laser that operates robustly at a frequency specified for DWDM systems, with the operating frequency independent of environmental variations such as temperature and humidity. Ideally, this laser should also be tunable among many or all of the DWDM channels, and it should be inexpensive and easy to produce and test.